Fewer mosquitoes testing positive for virus in Cook, DuPage counties

For those wondering when the threat of the West Nile virus will be over in Illinois this year, Dr. Linn Haramis has a ready answer:

"I usually plant impatiens flowers in front of my house, and they're annuals," said Haramis, program manager of vector control for the Illinois Department of Public Health. "When they're killed by a hard freeze, then the mosquitoes are dead too."

In other words, it's not over yet. But chances are Illinois is getting closer to the end of the West Nile virus season.

Although human cases are generally reported into October, officials think the numbers may have peaked, based on historic trends.

At the beginning of August, nearly 60 percent of mosquito pools tested in Cook County and 71 percent of those tested in DuPage County were positive for the virus. By the end of August and the beginning of September, those percentages dropped to about 32 and 30 percent. From Aug. 28 through Tuesday, they were about the same, at 26 percent and 33 percent, according to Haramis.

Officials worry when the positives come in higher than 10 percent.

As of Wednesday, Illinois had reported 92 cases and three deaths, two in DuPage and one in Kane County. Suburban Cook County reported an additional death, bringing the total to at least four. In 2011, there were 34 cases and three deaths.

Nationwide, the number of human West Nile cases through Tuesday reached 2,636, the highest through the second week of September since 2003. Meanwhile, the number of neuroinvasive cases, such as meningitis or encephalitis, reached 1,231, the highest through the second week of September since 1999, when the virus was first detected in the U.S.